<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>数字补0</title>
    <script>
        var num = prompt('请输入数字')
        var result = num < 10 ? '0' + num : num
        alert(result)
    </script>
</head>
<body>
    
</body>
</html>